1. Technical Field
The present disclosure relates to a photodetector with integrated microfluidic channel and to the manufacturing process thereof.
2. Description of the Related Art
As is known, today there are available numerous diagnostic devices, which find use, for example, in the biological field. In addition, in the field of diagnostic devices, it is known to use photodetectors of an integrated type, each of which includes at least one microfluidic channel optically coupled to a photodiode.
By way of example, the Italian patent application No. TO2012A000501 filed on Jun. 8, 2012in the name of the present applicant (corresponding to U.S. Patent Application No. 13/902,468) describes a photodetector comprising a photodiode, which is formed by a body of semiconductor material and an integrated optical structure, which is arranged on top of the semiconductor body. In addition, the photodetector comprises a microfluidic channel arranged on top of the integrated optical structure. The microfluidic channel houses at least one detection region and is designed to receive a first radiation having a first wavelength. The detection region includes at least one receptor, which is able to bind to a corresponding target molecule, in the case where the latter is present within the microfluidic channel. In turn, the target molecule can bind to a corresponding marker, which, when excited by the first radiation, emits a second radiation having a second wavelength, this second radiation being detectable by the photodiode.
In practice, the patent application No. TO2012A000501 describes a so-called “fluorescence diagnostic device”, which is characterized precisely by the use of markers that, when excited with a light radiation at a certain wavelength λe, emit a light radiation of their own at a wavelength λf greater than the wavelength λe. Consequently, by detecting with the photodiode the light radiation at the wavelength λf, it is possible to derive information on the chemico-physical characteristics of the specimen to be analyzed, which is made to flow within the microfluidic channel. In fact, the light intensity detected by the photodiode is a function of the amount of activated markers, this amount being a function of the number of target molecules.
In particular, the patent application No. TO2012A000501 describes the use of a Geiger-mode avalanche photodiode (GM-APD), also known as “single-photon avalanche diode” (SPAD), in so far as it is able to detect individual photons.
In general, a SPAD is formed by an avalanche photodiode and hence comprises a junction, typically of a P+/N type, or else N+/P type. The junction has a breakdown voltage VB and is biased, in use, with a reverse-biasing voltage VA higher in modulus than the breakdown voltage VB, typically higher by 10-20%. In this way, generation of a single electron-hole pair, following upon absorption of an photon impinging upon the SPAD, is sufficient for triggering an ionization process that causes an avalanche multiplication of the charge carriers, with gains in the region of 106 and consequent generation in short times (hundreds of picoseconds) of the avalanche current. The avalanche current can be collected, typically by means of an external circuitry connected to the junction and including anode and cathode contacts and forms an electrical signal at output from the SPAD.
The gain and likelihood of detection of a photon, i.e., the sensitivity of the SPAD, are directly proportional to the value of reverse-biasing voltage VA applied to the SPAD. However, the fact that the reverse-biasing voltage VA is appreciably higher than the breakdown voltage VB causes the process of avalanche ionization, once triggered, to be self-sustaining Consequently, once triggered, the SPAD is no longer able to detect photons, with the consequence that, in the absence of appropriate remedies, the SPAD manages to detect arrival of a first photon, but not arrivals of subsequent photons. In order to be able to detect also these subsequent photons, the avalanche current generated within the SPAD can be quenched, stopping the avalanche ionization process. In practice, one can reduce, for a period of time known as “hold-off time”, the effective voltage Ve across the junction, this effective voltage Ve coinciding with the reverse-biasing voltage VA only in the absence of photons, i.e., in the absence of current in the SPAD. In this way, the ionization process is inhibited and the avalanche current is quenched; then, the initial conditions of biasing of the junction are restored so that the SPAD is again able to detect photons. In order to reduce the effective voltage Ve across the junction following upon absorption of a photon, SPADs adopt the so-called quenching circuits, whether of an active type or of a passive type.
Irrespective of the details of implementation of the SPAD and thanks to the use of the latter, the diagnostic device described in the patent application No. TO2012A000501 is characterized by a high sensitivity. However, according to the patent application No. TO2012A000501, the microfluidic channel is formed on top of the passivation region and of the contacts of the photodiode. Moreover, a Bragg grating is present between the microfluidic channel and the photodiode, in order to increase the intensity of the electrical field inside the microfluidic channel. For these reasons, the optical coupling between the microfluidic channel and the photodiode may not be optimal, in particular for certain wavelengths.